A tanker truck at an oil and gas production site in southeastern Oklahoma.

Joe Wertz / StateImpact Oklahoma

Oklahoma experienced a dramatic drop in earthquakes in 2017 — a decline likely due, in part, to regulations limiting activity at oil-field disposal wells, scientists and experts say. New research suggests those regulations might be reducing some quakes more than others.

It’s been two years since state oil and gas regulators adopted a broad regional plan that limits the amount of wastewater pumped into disposal wells in quake-prone areas. The good news: It appears to be working. After peaking in 2015, earthquakes became a lot less frequent.

The troubling news: The next year, Oklahoma had a pair of strong earthquakes, including the largest ever recorded, the 5.8-magnitude Pawnee quake.

“The spatial relationship with the small magnitude earthquakes is being affected by the volume reductions, but not necessarily the large magnitude earthquakes,” says Ryan Pollyea, an assistant professor at Virginia Tech University who studied this disparity in a January paper published in the journal Geology.

Pollyea’s paper suggests pressure on smaller faults tapers off faster than pressure on larger faults — the ones that produce stronger shaking when they slip. The research also linked wastewater disposal to earthquake activity up to 75 miles away. That finding could mean a good practice for limiting larger earthquakes could be much larger volume limits in a lot more oil fields.

Oklahoma earthquakes also appear to be concentrated at certain depths. The injection of wastewater into crystalline “basement” rock has long been considered a risk factor for triggering shaking and is disallowed in permits issued to disposal wells operators.

In a separate paper published in the journal Science, a different team of researchers found that limiting the depth of injection wells might also reduce the frequency of earthquakes.

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A key part in solving the state’s earthquake crisis is the long-term management of an enormous amount of oil-field wastewater likely triggering the shaking. The energy industry is working to solve this billion-barrel-a-year problem, and one promising alternative to risky disposal wells is reusing wastewater instead of pumping it underground.

2017 is wrapping up, but the growing group of reporters at StateImpact are following many important government policy issues that will carry on into the new year.

Senior Reporter and Managing Editor Joe Wertz brought the StateImpact team into the studio for a preview of their coverage in the year to come. Here are some excerpts from the conversation edited for clarity:

A new research paper suggests Oklahoma’s earthquake hazard might not taper off as quickly or as significantly as scientists previously predicted.

The energy industry practice of pumping toxic waste-fluid byproducts of oil and gas production into underground disposal wells is thought to be fueling Oklahoma’s earthquake surge. This activity peaked in 2015 and slowed due to regulations and low oil prices.